The present invention relates to the processing of optical signals and, more particularly, to delaying optical signals.
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. One mechanism for the transmission of analog optical signals is through the use of some sort of pulse modulation, where a stream of optical pulses is modulated by an analog signal. Pulse Position Modulation (PPM) is a well-known modulation technique for radio-frequency transmissions. It is also used in analog optical communications. In PPM, a shift in the position of each pulse represents a sample of the original analog signal. Since the pulse repetition frequency (PRF) of the optical pulses must be greater than twice the bandwidth of the analog signal to correctly sample the analog signal, PRFs for optical communications will be quite high. For example, an optical inter-satellite link designed to transmit waveforms with a bandwidth of 20 GHz requires a PRF of over 40 GHz.
The optical pulses within the stream should be of short duration, since it is well known in the art that PPM signal-to-noise ratio (SNR) performance improves as the pulse widths within the modulated pulse stream decrease. Pulse widths as short as 0.3 picoseconds may be desirable for a PPM optical communication system. However, is also well known in the art that PPM performance will suffer if the shapes of the optical pulses vary or the amplitudes of the pulses vary on a pulse-to-pulse basis. Mode locking of a pulsed laser is a mature technique for producing equally spaced ultra-short identical pulses. It would be beneficial to use a mode-locked laser in a PPM communication system if the equally-spaced pulses produced by the system could be modulated without distortion.
Therefore, implementations of PPM for optical communications require a mechanism for modulating the delays between extremely short optical pulses within a pulse stream without modulating the shapes or pulse-to-pulse amplitudes of the pulses. Direct modulation of a semiconductor laser will appropriately modulate the delay between the optical pulses generated by the laser. However, a directly modulated semiconductor laser generates relatively long pulses that result in limited SNR performance. Pulse compression can be used on the longer pulses produced by the directly modulated semiconductor laser, but devices to provide such compression are complex and cumbersome. Direct modulation of a semiconductor laser may also introduce amplitude modulation or pulse reshaping of the individual time-shifted pulses, further limiting performance.
Pulse position modulation of extremely short optical pulses is also achieved by applying a pulse-to-pulse delay external to the source of the equally spaced optical pulses. That is, a method and apparatus are used that can receive a stream of optical pulses, change the pulse-to pulse delay at the rate required for properly sampling the transmitted analog signal, and further transmit the delayed pulses. It is known in the art that one example of a pulse position modulator for optical pulses consists of an optical delay line, such as a parallel slab of transparent electro-optically active material. The refractive index of the electro-optically active material can be controllably varied by an applied voltage, so that each pulse is controllably delayed upon traversing the electro-optically active material in accordance with the instaneous voltage. However, such a modulator requires an undesirably large amount of electrical power, due to the relatively large voltages required to modulate the refractive index of the material and thus modulate the delay encountered by a pulse traversing the material.
Another example of a pulse position optical modulator relying upon the use of electroptically active material is disclosed in U.S. Pat. No. 3,961,841, issued Jun. 8, 1976 to Giordmaine. Giordmaine discloses a device for optical pulse position modulation comprising a diffraction grating in combination with an electro-optic prism and a lens. The diffraction grating splits an incident light pulse into its frequency components and the lens directs the components into the prism. The refractive index change provided by the prism causes a phase shift in the frequency components and thus a time shift in the optical pulse once it is reconstructed by the diffraction grating. The device disclosed by Giordmaine provides the capability of modulating light pulses as short as one picosecond. However, the maximum controllable delay is limited to a few picoseconds for a 3 picosecond pulse and further decreases for shorter pulses. Also, the multiplicity of optical elements such as the diffraction grating, lens, and prism increase the complexity and manufacturing cost of the device.
A device for delaying optical pulses is disclosed in U.S. Pat. No. 5,751,466, issued May 12, 1998 to Dowling et al and is shown in FIG. 1. Dowling discloses a photonic bandgap structure comprising a plurality of cells 18A-18N of width d in which the refractive index varies. The refractive index variation may be such that each cell comprises two layers of materials with two different indices of refraction n1 and n2. If the widths of the two layers within each cell are xcex/4n1 and xcex/4n2 where xcex is the free space wavelength of the optical pulse to be delayed, a distributed Bragg reflector structure is created. According to Dowling, the thickness and/or number of layers in the photonic bandgap structure and/or their indices of refraction are selected to produce a structure with a transmission resonance center frequency and bandwidth corresponding to the frequency and bandwidth of the optical pulse to be delayed. By matching the transmission resonance to the optical pulse, a controllable delay is imparted to the optical. pulse without significantly altering the optical signal.
The device disclosed by Dowling requires that the thickness of each layer in the device be approximately one-half the wavelength of the incident optical pulse to form the photonic bandgap structure. The delay imparted on an optical signal by transmission through the structure will depend upon the number of layers and the indices of refraction within the layers. The structure can be thought of as essentially increasing the length of the waveguide in which it is contained, thus providing the desired delay. For example, Dowling discloses a simulation of a photonic bandgap structure that is 7 xcexcm thick that provides a delay equivalent to an optical signal traveling through a 110 xcexcm stucture, or a delay of about 0.4 picoseconds. Since the amount of delay from a single structure is relatively small, Dowling discloses that the structures can be successively coupled in a single device to provide additional delay. Of course, this increases the overall size of the device.
Dowling also discloses that the delay provided by a photonic bandgap structure can be varied by changing the indices of refraction within the layers of the structure. One way to accomplish this is to fabricate at least one of the layers from electro-optically active material. An applied voltage will then change the index of refraction in the layer to which the voltage is applied. FIG. 1 shows a voltage means 15 that applies a voltage to one or more of the layers within the device disclosed by Dowling. Varying the voltage would vary the delay, thus providing the controllable delay required for pulse position modulation. However, since the overall delay provided by photonic bandgap structure is relatively small, it would follow that the change of delay provided by electro-optically changing the indices of refraction would only be some fraction, typically 0.1% or less, of that relatively small delay. Again, this limitation could be overcome by coupling successive structures, with a corresponding increase in the overall size of the structure.
There exists a need for a high quality optical delay apparatus and method that provide large, controllable delays for short optical pulses. Moreover, the apparatus and method must be capable of providing the required delay without substantially altering the pulse-to-pulse amplitude or shape of the pulses in the original pulse stream. Additionally, it is important for the delay generation to be implemented in a compact, lightweight apparatus that is compatible with other integrated systems.
Accordingly, it is an object of the present invention to provide apparatus and methods for optical delay generation.
It is another object of the present invention to provide apparatus and methods for optical delay generation without causing pulse-to-pulse amplitude modulation or pulse reshaping of delayed optical pulses.
It is another object of the invention that the method and apparatus provide optical delay that can be used for pulse position modulation.
These and other objects are provided according to the present invention by transmitting optical pulses to be delayed into a waveguide means comprising electro-optically active material within which is formed a chirped distributed Bragg reflector (C-DBR) oriented in the direction of light propagation within the waveguide means. The chirped distributed Bragg reflector reflects light at different wavelengths at different points within the waveguide. An electric field generator generates and controls an electric field applied across the waveguide in a direction perpendicular to the direction of propagation. Changes in the electric field intensity cause changes in the index of refraction within the waveguide means, thus changing the point at which the optical pulses reflect from the chirped distributed Bragg reflector and are transmitted out of the waveguide means. Thus, optical delay generation is accomplished by controlling the intensity of the electric field across the chirped distributed Bragg reflector.
In a first specific embodiment of the present invention, the waveguide means comprises a straight waveguide constructed from electro-optically active material, such as lithium niobate, sandwiched between a top conductor and a bottom conductor. A chirped distributed Bragg reflector is formed in the waveguide by quasiperiodically corrugating the waveguide walls. A voltage source is connected to the top conductor and the bottom conductor such that a voltage between the two is created. The voltage causes an electric field to be generated across the chirped distributed Bragg reflector, thus changing the index of refraction as the voltage changes. An alternate embodiment uses a tapered waveguide in which the waveguide walls are periodically corrugated.
In a second embodiment of the present invention, the electro-optically active material used in the waveguide means comprises a semiconductor chirped distributed Bragg reflector structure with excitonic band just above the photon energy structure. The chirped distributed Bragg reflector is formed by the individual layers of semiconductor material. The refractive index and thickness of each layer vary from its neighbor so as to provide the quasiperiodic variation in refractive index required to form a chirped distributed Bragg reflector.
The chirped distributed Bragg reflector of an alternative embodiment of the present invention comprises an apodized chirped distributed Bragg reflector. Apodization of the chirped distributed Bragg reflector reduces the oscillations in the group delay of the optical pulse that would result if the optical pulse were reflected by a linearly chirped distributed Bragg reflector. Hence, distortion of optical pulses is reduced.
Reflection of optical pulses from a chirped distributed Bragg reflector results in broadening of the optical pulses due to an acquired chirp. Therefore, in another embodiment of the present invention, the time-delayed pulses output from the delay generator are passed through a dispersion compensating fiber, which provides correction for the acquired chirp.
The present invention is used to provide optical pulse position modulation for an analog signal. A stream of equally-spaced optical pulses is transmitted into a waveguide containing a chirped distributed Bragg reflector. The analog signal controls a modulation means that generates an electric field across the waveguide. The modulation means controls the intensity of the electric field and thus the delay provided by the waveguide. Each optical pulse in the stream of optical pulses is reflected by the chirped distributed Bragg reflector and acquires a delay corresponding to the analog signal.